1. Field of the Invention
The present invention relates to a color correction device and method which adjust the color components of an image signal containing color information and a color electrophotographic machine which employs such a color correction device.
2. Description of the related art
In color electrophotographic machines, there are presently available digital color copying machines. The digital color copying machines optically read an image in an original to obtain separate electrical image information signals on three primary-color images of R (red), G (green) and B (blue). These information signals are converted to different electrical information signals of C (cyan), M (magenta) and Y (yellow). The respective basic color images are printed by C, M and Y toners on the basis of the different electrical image information signals of C, M and Y, and the original image is reproduced by laying these printed colors one on top of another.
Theoretically, the original image and print image are the same in color. However, actually, if the R, G and B signals obtained from the original are solely converted to C, M and Y signals and are printed in three color toners, a large difference will be produced between the original color and the printed color because (a) the spectral characteristic of color separation filters used in the image read unit are actually not ideal, (b) the color of each of the toners used for printing is also not ideal, and (c) the color obtained by the principle of subtractive color mixture for laying a plurality of print colors sequentially one on top of another is different from the actual printed color. In order to avoid such discrepancy, when the R, G and B signals are converted to the C, M and Y signals, a masking process is performed to correct the colors. Furthermore, in order to reproduce a correct achromatic color, often achromatic color components (black and gray) are extracted from the R, G and B signals and are printed by a separate black toner. However, an "additivity rule of densities" does not actually hold due to the influence of surface reflection, etc. As a result, correct color correction cannot be performed.
The masking techniques are known, for example, in (1) Japanese Laid-Open Publication No. 59-161981 (British Patent No. 2145598) and (2) Japanese Laid-Open Publication No. 62-47273. In the techniques of the reference (1), nine coefficients used in the masking process are separately adjustable in a plurality of stages. By changing those coefficients, the contents of the color correction by the masking process are changed and as a result the colors to be printed are adjustable. In this technique, however, since the respective masking coefficients do not correspond to the basic colors of Y, M and C in one-to-one relationship, it is not clear which coefficients should be adjusted in what manner in order to obtain a desired particular color. Therefore, adjustment is required to be repeated many times by trial and error. If the color tone of a particular color is preferentially changed, a balance among Y, M and C colors may be undesirably lost and achromatic color portions such as black and gray portions of the image may often be colored.
The techniques of the (2) reference are intended to correct the colors by referring to the contents of a memory table and changing a part or the whole of the memory table to adjust the colors. However, in the particular reference, it is necessary to prepare a memory having a large capacity corresponding to the degree of freedom of color adjustment. In that case, therefore, if the degree of freedom of color adjustment is heightened, the device would undesirably become expensive.
In these types of devices, the user must adjust colors as needed in addition to the basic color correction because of the aging of the color characteristic of the device itself, variations in the characteristic of the individual devices, the user's own taste in colors, and intentional requirement for color change, etc.